High cis-1,4-polybutadiene can be prepared by polymerizing 1,3-butadiene monomer with nickel based catalyst systems. Such nickel based catalyst systems contain (a) an organonickel compound, (b) an organoaluminum compound, and (c) a fluorine containing compound. Such nickel based catalyst systems and their use in the synthesis of high cis-1,4-polybutadiene is described in detail in U.S. Pat. No. 3,856,764, U.S. Pat. No. 3,910,869, and U.S. Pat. No. 3,962,375.
The high cis-1,4-polybutadiene prepared utilizing such nickel based catalyst systems typically has a high molecular weight. Due to this high molecular weight, the high cis-1,4-polybutadiene is generally oil extended. However, this precludes the high cis-1,4-polybutadiene from being utilized in many applications. For instance, such oil extended rubbers cannot be utilized in tire sidewalls which contain white sidewall compounds. In any case, there is a large demand for high cis-1,4-polybutadiene having a reduced molecular weight which can be processed without being oil extended.
Various compounds have been found to act as molecular weight reducing agents when used in conjunction with the nickel based catalyst system. For instance, Australian Patent 556,294 discloses that alpha-olefins, such as ethylene and propylene, act as molecular weight reducing agents when utilized in conjunction with such three component nickel catalyst systems. Canadian Patent 1,236,648 indicates that 1-butene, isobutylene, cis-2-butene, trans-2-butene, and allene act as molecular weight regulators when used in conjunction with such nickel based catalyst systems. U.S. Pat. No. 4,383,097 reveals that certain nonconjugated diolefins, such as 1,4-pentadiene, 1,6-heptadiene, and 1,5-hexadiene, act as molecular weight reducing agents when utilized in conjunction with such catalyst systems.
The processibility of high cis-1,4-polybutadiene rubbers can be improved by simply lowering their molecular weight. However, this approach also typically leads to increased cold flow. Accordingly, the use of conventional molecular weight reducing agents to improve rubber processibility leads to compromised cold flow characteristics.